Changes in cell shape drive morphogenesis and actin deficiencies cause failed cell shape change and birth defects. Yet our knowledge of actin's influence on morphogenesis is largely limited to lists of molecules and descriptions of gross phenotypes. While actin filaments (F-actin) are recognized as the major structural determinant of cell shape, basic questions remain unanswered: How is F-actin remodeled? How does it inter- face with cell membranes? How does it adapt to environmental or genetic variability? We need these answers to understand development and human disease. Our long-term goal is to fully understand actin's role in forming healthy tissues. We are focused on Drosophila cellularization, the process that builds the first epithelial sheet in the embryo. During cellularization, three cell surface remodeling events are going on in every new cell of the embryo: (i) microvilli disassemble; (ii) a membrane furrow ingresses; and (iii) an actomyosin ring contracts. We have shown that while these three remodeling events are spatially segregated over tens of microns, they are still kinetically coupled to each other and to measurable changes in the actomyosin ring. We have also identified specific F-actin regulators in the actomyosin ring that promote the robustness of cellularization against environmental and genetic perturbation. These results argue that actin does more than just give cells their shape. Our working model is that actin- based mechanisms also contribute to the coordination, kinetics, and robustness of morphogenesis. In this proposed work, we will test our model as follows: In Aim 1, we will test the hypothesis that plasma membrane tension, generated by furrow ingression, suppresses F-actin polymerization in microvilli, causing them to unfold or fall apart. In Aim 2, we will test he hypothesis that distinct kinetic phases of actomyosin ring contraction are governed by distinct mechanisms of F-actin regulation. In Aim 3, we will test the hypothesis that F-actin regulators, Srya and Spt, stabilize F-actin within the contractile ring to ensure robust cellularization in the face of environmental and genetic perturbation. We have assembled a multidisciplinary team of physicists, mathematicians and technologists; and we will employ a powerful combination of approaches, including quantitative live imaging, physical force assays, genetics, and genome editing to examine the events of cellularization in uncommon detail. Because the actin- based machinery that drives cellularization is widely conserved, our findings will be relevant to other organ- isms, including humans. We will establish how individual remodeling events are regulated in space and time, while also showing how the events are coordinated with each other to accomplish cellularization at high fidelity. In doing this work, we will also reveal how genes, mechanics, and environment regulate F-actin.